I'm sorry the game is giving you trouble Direwolf! If your able to load the game up until stage start that is a good sign, comparatively to older hardware that can't boot the main menu.
Enemies do despawn off-screen and we recently reduced the AI calls which lightens the load on scripts. Next goal to unload stage modules and a lower texture option, we have packed all our sprites onto optimized sprite sheets with limited colors, may be a limit on how far we can push the game on older systems.